As described in patent document 1, a technique for enhancing an operational stability of a vehicle is being developed. Other than patent document 1, other techniques for enhancing the operational stability of the vehicle can be considered, one of which is to enhance the operational stability of the vehicle by placing a relatively thick washer at a fastening portion. The inventor of the present invention focused on such technique, and conducted an experiment to check how the operational stability changes when the washer is incorporated at the fastening portion.
Specifically, the operational stability was checked for when the washer was not used, when a washer having a thickness of 4 mm was used, and when a washer having a thickness of 8 mm was used. According to the experiment, the operational stability was poor when the washer was not used, and the operational stability was relatively poor when the washer having the thickness of 4 mm was used. On the contrary, satisfactory operational stability was obtained when the washer having the thickness of 8 mm was used.